Since modern brake systems along with other fluid pressure operable accessories employed on present day motor vehicles, particularly tandem or tractor-trailer type vehicles equipped with dual brake circuits, demand ever increasing compressed air or fluid pressure supply for operation thereof, it becomes increasingly difficult and impractical to provide a fluid pressure storage reservoir of adequate volume for accommodating such demand. A proposed solution to the problem has been to provide compressed air or fluid at a higher pressure level and thereby reduce the volume and size of the storage reservoir. Since the brake system itself, however, cannot operate at the higher level of pressure, such pressure must be limited or reduced to a degree commensurate with that required by the brake circuits.
Brake control valve devices for vehicles having dual brake circuits, in which manipulation of an operating lever or foot pedal with consequent actuation of a pressure graduating piston and a relay piston effects operation of inlet and outlet valves for respective brake circuits, are presently known. Such a valve device essentially comprises a first mechanically operable valve element for effecting supply of operating pressure to a first brake circuit and a second valve element operable in series with said first valve element, through a lost motion connection, said second valve element thereafter being controlled by a relay piston actuable responsively to operating pressure supplied to said first brake circuit. Since the relay piston (comprising two telescopically arranged concentric pistons), for the purpose of limiting the foot pedal force (upon dropping out of the first brake circuit) has to be of relatively large dimension to obtain a tolerable synchronized operation of the two brake circuits, the overall dimensions of the brake valve device itself become relatively large. Moreover, since such a device employs rubber type pads as part of the return spring arrangement, the level or degree of brake pressure supplied to the respective brake circuits is difficult to reduce or limit due to the characteristic progressive rate of compressibility of the rubber pads. Thus, the use of rubber type pads in such valve devices becomes somewhat impractical, notwithstanding that the use thereof would be desirable.